Resonator



May 7, 1963 w. P. ASTEN 3,382,459

RESONATOR Filed May lO, 1965 2 Sheets-Sheet l E'IGJ.

INVENTOR WILLIAM P. ASTEN W. P. ASTEN 3,382,459

RESONATOR 2 Sheets-Sheet 2 May 7, 196s Filed May lO, 1965 I O 6 5 IIIII II |I| II. II 2 9 M f .JWM S I G N WW I /Vh\ L n 6 .f \\w\ y Ill 3 n I|\\\\ 3 w I I G I II I H lllll I1 I I I IIIII II III I I r w WILLIAM R ASTEN BY ATTORNEYS United States Patent O 3,382,459 RESNATOR William P. Asten, Aldie, Va., assigner to Melpar, Inc., Falls Church, Va., a corporation of Delaware Filed May 10, 1965, Ser. No. 454,311 29 Claims. (Cl. 331-456) ABSTRACT F THE DESCLOSURE An electromechanical resonator includes a tuning fork having a U-shaped configuration in which normally parallel tines are connected by a bridging portion, the fork supported along one of the tines only, to permit freedom of vibration of that tine along its unrestricted length and of the other tine and the bridging portion along their entire lengths. The fork is driven in either of the tuning fork (opposite movement of the tines) and reed (common movement of the tines) modes of vibration at the disparate natural frequencies of those vibrational modes, and separate and distinct detectors are utilized for sensing the vibrations of the fork in the respective modes.

The present invention relates generally to resonators of the tuning fork type, and more particularly, to a tuning fork type resonator wherein one tine is anchored so a second tine and a leg joining the tines together are free to vibrate so that substantial vibrations in two different modes, at distinct frequencies, are established.

Resonators of the tuning fork type are capable of vibrating in two separate modes, generally at different natural frequencies, namely in the reed mode and the tuning fork mode. In the latter vibration mode, the tines generally have equal, but opposite velocities relative to their rest position, i.e., during one half cycle of oscillation, both tines are directed toward their rest position, while during the other half -cycle of oscillation they are directed away from the rest position. When a tuning fork vibrates as a reed, in contrast, one tine vibrates toward its rest position while the other tine moves away from the rest position.

In prior art tuning fork assemblies in which the leg joining the two tines together is fixedly mounted, the Q of the resonator when it vibrates as a fork is generally on the order of two magnitudes greater than the Q of the vibrations of the fork assembly when it vibrates as a reed. The disparity between the Qs in the two different oscillating modes has caused tuning fork designers considerable difliculty.

The difficulty arises if the tuning fork is utilized as a frequency sensitive relay or as a filter, because low am plitude signals are coupled through the fork at the reed frequency. Because only a single pickup device is employed for both fork and reed vibration modes in all prior art devices, it is frequently impossible to distinguish between filter outputs at the two frequen-cies associated with the different modes. Another problem associated with prior art electromechanical tuning fork resonators, when employed as filters, is that fork mode vibrations result in the output pickup element deriving a signal 180 out of phase from oscillations occurring at the reed frequency relative to the input. In consequence, even if it were desired to pass both the reed and fork natural frequencies, the pickup voltages are of reversed relative polarity so that for phase detection applications, the information coupled through the filter is of little value.

According to the present invention, there is provided an electromechanical resonator of the tuning fork type, wherein one tine of the fork is anchored. By anchoring one tine of the fork, the other tine is free to vibrate as is the leg that connects the two tines together. Since the leg connecting the two tines together is free to vibrate, the fork oscillates in both the reed and tuning fork modes. The Qs of oscillations in the two modes are separated from each other only by a factor on the order of 10 Thereby, the amplitude of the signals derived from pick ups for the two different modes of operation can be made approximately the same.

Because one tine of the tuning fork is anchored, separate pickup members can be employed for detecting the reed and tuning fork vibrations. The use of two different output sensors is ideally adapted for use as a dual frequency sensitive relay wherein one of two outputs is activated depending upon the excitation frequency applied to the tuning fork. Also, the use of two separate and distinct pickup devices enables the tuning fork to be employed as a dual frequency band pass filter in which signals at both frequencies are of substantially the same phase and relative amplitude.

A further feature of the present invention resides in its use as a very stable oscillator at two different frequencies. I have found that oscillations at both the reed and fork frequencies can be attained with the present invention by reversing drive and pickup transducers, disposed in proximity to the free tine and the leg joining the tines together. A single high-gain positive feedback ampliiier has its input and output selectively connected to the transducers. It is merely necessary, when using the present invention as a dual frequency oscillator, to reverse the polarity of the signals coupled to the amplifier to derive one or the other of the two frequencies.

It is, accordingly, an object of the present invention to provide a new and improved tuning fork configuration.

Another obje-ct of the present invention is to provide a tuning fork configuration wherein reed and fork modes of vibration are at Qs on an order of magnitude different from each by approximately one.

It is another object of the present invention to provide a new and improved resonator of the tuning fork type wherein two separate and distinct frequencies can be detected with approximately the sarne capability.

An additional object of the present invention is to provide an electromagnetic resonator of the tuning fork type wherein one tine of the fork is ancho-red, while the other tine and the leg joining the two tines are free to oscillate so that substantial vibrations in both the tuning fork and reed modes are attained.

Still another object of the present invention is to provide a tuning fork configuration in which separate pickup elements are employed for detecting the reed and fork modes of vibration.

A further object of the present invention is to provide a new and improved oscillator having an electromagnetic resonant element that is susceptible to oscillation at two different frequencies and wherein the oscillator derives two distinct output frequencies dependent upon the resonator oscillation mode.

Still a further object of the present invention is to provide a new and improved dual frequency oscillator employing a tuning fork that is susceptible to vibration both in the tuning fork and reed modes wherein the vibrations are approximately on the same order of magnitude, so that the same amplifier can be employed for the dual frequency output and only switching is necessary to change from one frequency to another.

Still another object of the present invention is to provide a new and improved relay that is capable of deriving a pair of separate outputs in response to two different frequencies applied thereto.

Still a further object of the present invention is to provide a dual frequency responsive relay employing a resonator that is capable of oscillating in both the reed and tuning fork modes and wherein contact is made at two different points depending upon the frequency applied to the relay.

Still a further object of the present invention is to provide a new and improved filter employing an electromechanical resonant element from which can be derived outputs at two different frequencies, both having the same phase relative to the input signal.

Still an additional object of the present invention is to provide a new and imp-roved dual frequency filter employing a ,relatively high Q electromechanical resonating element that oscillates at two different frequencies with amplitudes approaching the same order of magnitude.

Yet an additional object of the present invention is to provide a dual frequency lter employing an electromechanical resonator which is free to vibrate both in the reed and fork modes and wherein outputs are derived from distinct pickup elements for the two oscillation modes.

The above and still further objects, features and advantages of the present invention will become apparent upon `consideration of the following detailed description of one specific embodiment thereof, especially when taken in conjunction with the accompanying drawing, wherein:

FIGURE l is a schematic representation of the tuning fork according to the present invention in combination with an amplifier so that oscillations at two distinct frequencies can be derived;

FIGURE 2 is a schematic diagram of the manner in which the present invention is employed as a dual frequency sensitive relay;

FIGURE 3 is a schematic diagram illustrating the manner in which the tuning fork configuration of the present nvention is employed as a dual frequency bandpass lter;

FIGURE 4 is a schematic representation of a further embodiment of the present invention;

FIGURE 5 is a schematic representation to illustrate the manner in which the electromechanical resonator of the present invention vibrates in the tuning fork mode; and

FIGURE 6 is a schematic illustration to show the manner in which the electromechanical resonator of the present invention vibrates as a reed.

Reference is now made to FIGURE 1 of the drawings wherein electromechanical resonator 11, shaped as a sheet metal tuning fork and preferably made of a highly magnetically permeable material such as Ni-Span-C, has rst and second tines 12 and 13, joined together by leg 14. Tine 13 is somewhat longer than tine 12 and is anchored at its end remote from leg 14 by means of bolt 15 that extends into stationary housing 16. A circular aperture or slot, neither of which is shown, is provided in tine 13 at its end so that bolt 1S can anchor tuning fork 11 at only one position or a plurality of different positions. If it is desired to change the frequencies derived from the unit by other than conventional means, that is by varying the fork mass and/or dimensions, a slot is provided in tine 13 and fork 11 is moved relative to Ibolt 15 and housing 16.

Positioned in proximity to leg 14 is a first coil or velocity-electric signal transducer 17 having extending through it bar magnet 18. At the end of tine 12 remote from leg 14 is a second coil or velocity-electric signal transducer 19 having bar magnet 20 extending through it. Bar magnets 1S and 20 are arranged to have their opposite poles in closest proximity with tine 12 so tine 12 is provided with a DC magnetic bias to provide low reluctance for the AC signals coupled between coils 17 and 19.

Before .considering .the manner in which the network of FIGURE 1 functions as a dual `frequency oscillator, consideration will be given to the manner .in which electromechanical resonator 1f1 vibrates both in the reed and fork modes, by referring to FIGUR-ES 5 and `6. in FIG- UR'E 5, it is assumed that a frequency F1, coincident with the natural resonant frequency of the resonator vin its fork mode of vibration, is applied :to coil 17. The force applied to fork 11 by the electromagnetic field deriving from coil 17 causes vibration of the fork substantially in the tuning fork mode wherein leg 14 that joins tines 1-2 and 13 together remains substantially at the same position throughout an entire period of oscillation. The free end of tine 12, however, oscillates with substantial excursions `between its rest position and its maximum eX- tent of travel, as indicated by dotted lines 22 and 23.

Since the end of tine 13 is anchored to mounting post 1'6, no substantial movement of tine y13 occurs as tine l2 oscillates. What movement there is of tine 13 at points in proximity to leg 14, is always oppositely directed relative to the center line between tines 12 and 13 from the movement of tine 12 at its free end. Thus, at an instant of time when the free end of tine '12 extends most inwardly, as indicated by dotted line 23, the portion of tine 13 from bolt 15 to leg 1-4 extends inwardly, as indicated by dotted line 24, towards the center line joining the tines together, Similarly, during the other half cycle of tuning fork vibration, tines |12 and 13 both move away from the center line, as indicated by dotted lines 22 and 2S.

`It is noted that there exists a substantially stationary point 26 between driving coil 17 and the free end of tine 12. Stationary point 26 is -a point -about which the oscillations are derived, similarly to anchoring leg 14 in a convention-al tuning fork. Because there are unequal masses of fork 11 relative to stationary point 26, the Q of the oscillations derived in the tuning fork mode with the present invention are substantially lower than those obtained with the conventional prior art work. If it is desired t-o increase the Q of the resonator of the present invention, the fork is designed so that stationary point 26 is moved closer to leg 14, and oppositely, decreasing Qs are obtained by arranging fork 11 so stationary point 26 is closer to the free end of tine 12.

To provide an understanding of the manner in which the fork of the present invention vibrates in the reed mode, reference is made to FIGURE 6 wherein a frequency F2, different from frequency F1 and coincident with the natural resonant frequency of resonator 11 in the reed mode of oscillation, is applied to coil 17. In response to the electromagnetic energy at frequency F2 deriving from coil 17, fork 11 is caused to vibrate as a reed wherein the ends of tines 12 `and 13 remote from leg 14 remain in substantially the same position throughout a cycle of oscillation while leg 14 joining the tines together vibrates substantially. Thus, during one half cycle of oscillation at frequency F2, both tines 12 and 13 move to the left of their rest position as depicted by dotted lines 27 and 28, while during the other half cycle, the tines move to the right of their rest position, as indicated by dotted lines 29 and 30;

While the excursions in the vicinity o'f leg 14 when resonator -11 is excite-d to the reed mode of vibration are not as great as the excursions of the free end of tine 12 during the for-k mode of vibration, they are approximately on the same order of magnitude. The relative amplitudes of these vibrations are such that when resonator `11 oscillates in the tun-ing fork mode, Qs on the order of to 150 are obtained. This is not too difieren-t from `the Qs of approximately 50 for reed mode vibration-s. A

Because substantial movement occurs at two distinct points for the different modes of vibration of the present invention, namely at the free end of -tine 12 and in the vicinity of leg 14, it -is possible with the present invention to provide -two separate and distinct output detecting means. One ofthe detecting means is located in proximity to :the free end of tine 12 `and provides an output only when fork 11 is driven in the tuning fork mode at frequency F1. In contra-st, a pickup element in proximity to leg 14 is responsive only to reed vibration of fork 11 and provides an output when the fork is driven at frequency F2. Movement of leg 14 in response to fork inode vibrations is generally in-sufiicient to provide a large amplitude output from coil 17 at frequency F2 while displacement of the free end of t-ine 12 in response to reed vibrations is generally insufficient 4to provide a large amplitude signal from tnansducer 19. Since the resonator outputs for the two different vibration modes are on the approximately same order of magnitude and are derived from different transducers 17 and 19, the structure of the present invention is ideally adapted for use as a dual frequency oscillator, dual frequency filter, or dual frequency relay.

In use as an oscillator for deriving a pair of frequencies, the circuit diagram of FIGURE 1 is employed. In FIGURE 1, coil 17 is connected via stationary contacts 32 and 313 and movable contacts 34 and 35 to the output of amplifier 36. In an opposite manner, coil 1'9 is connected to the input of amplifier 36 through stationary contacts 37, 38 and movable contacts 39, 40. Switch contacts 34, 35, 39 and 40 are arranged so that coil v17 can be connected to the input of amplifier 36 while coil 19 is connected to the amplifier output.

By reversing the connection of switch contact-s 34, 35, 39 and 40, the polarity of the signal coupled between the amplifier output -and coil 19 is reversed from the polarity when the amplifier output is connected to coil 17. Thereby, a positive feedback loop is maintained for both connection situations, regardless of which coil is used for driver and pickup functions. Amplifier 36 is provided with sufiicient gain in both connection modes to be driven into saturation during every half cycle of its operation, whereby square waves are derived, in a manner well known to those skilled in the art. Thereby, stabili-ty of the oscillations derived from fork 1,1 is insured at both operating frequencies.

When it is desired to provide oscillations at frequency F1, coincident with the natural resonant frequency of fork 11 in its fork mode of vibration, ampli-fier 36 is connected to coils 17 and 19 as illustrated 4in FIGURE l. Any vibrations imparted to fork 11 at frequency F1 are picked up by coil 19, increased in magnitude by ampliter 36 and regeneratively coupled back -to coil 17, which serves as a drive coil. The F1 oscillations impar-ted by coil 17 to fork 11 result in regenerative oscillations for the Ifork at frequency F1 so that sta'ble oscillation las indicated 'by FIGURE 5 are quickly derived.

Activating the switch so that contacts 34 and 35 engage fixed contacts 37 and 38, respectively, while contacts 39 and 40 alight on contacts 32 and 33, respectively, changes the freqency at which fork 11 vibrates to frequency F2. This is because coil 19 now serves as a driver while coil 17 is utilized as a pickup whereby a regenerative path is provided through amplifier 36 at frequency F2. Oscillations of resonator 11 at frequency F2, result in that frequency being supplied to the input of amplifier 36. The F2 signal deriving from amplifier 36 is coupled to coil 19 in phase with the signal applied tothe amplifier. Since the oscillations applied to coil 19 are regenerative, fork 11 continues to oscillate at frequency F2.

Any tendency for fork 11 to vibrate at frequency F1 is degenerate with the switch reversely connected from the manner illustrated, and vice versa. Degeneration occurs because of the reversed phase with which the F1 and F2 signals are applied to coils 17 and 19 for the reversed and illustrated switch positions. An inspection of FIG- URE 5 indicates that frequency oscillations at frequency F1 as picked up in coil 17 are reversed in phase relative to those of frequency F2. When the fork is vibrating in the fork mode, leg 14 is closest to coil 17 when the free end of tine 12 is the farthest from coil 19. In contrast, an inspection of FIGURE 6 shows that tine 12 is closest to both coils 17 and 19 during the same portion of an oscillating cycle. Thus, the network of .FIGURE 1 can regeneratively oscillate at only one frequency, F1 or F2, at a time, and the other frequency is degeneratively fed back to preclude oscillation.

While the illustrated connection is most advantageous because it requires a minimum of pickups, a dual frequency oscillator can also be provided if coil 17 is a driver for both modes. In such an event. an additional pickup coil is provided in proximity to leg 14. To establish F1 or F2 oscillations, coil 19 and the additional coil are selectively connected to the amplifier input and its output is always connected to driver coil 17.

Reference is now made to FIGURE 2 of the drawings, a modification of the present invention, wherein an indication is provided as to whether variable frequency source 43 is of frequency F1 or F2. Source 43 is coupled to driver coil 17 to cause oscillation of fork 11 in the reed or fork modes, at frequencies F2 and F1, respectively. Mounted in proximity to leg 14, on tine 13, is contact pad 44 for selectively engaging fixed contact 45', that is mounted by insulating assembly 46 on housing 47. Similarly, contact pad 48 is mounted at the free end of tine 12 remote from leg 14 to engage selectively fixed contact 49 that is carried on housing 47 by insulating assembly 50. Fork 11 is grounded to metallic container 47 so that circuits can be established through the fork to contacts 45 and 49. Because of the higher Q of resonator 11 in the fork mode of vibration than in the reed mode, contact 49 is more distantly positioned from pad 48 than is contact 45 from pad 44.

To provide an indication as to whether source 43 is at frequency F1 or F2, contacts 45 and 49 are connected through separate indicating networks comprising batteries or DC sources 52 and 53, each of which is separately connected in series with indicator lamps 54 and 55. The activation paths for lamps 54 and S5 are completed by the connections to ground so that visual indications are derived when source 43 is at frequency F1 or F2.

In response to source 43 being of frequency F1, resonator 11 vibrates in the fork mode and contact is made between contacts 48 and 49 at frequency F1. The motion of the end of fork 11 where tines 12 and 13 are joined by leg 14 is insufficient, however, to provide engagement between contacts 44 and 45. Thereby, lamp 55 is activated to the exclusion of lamp 54 when fork 11 is driven at frequency F1. Similarly, but in an opposite manner, a frequency F2 deriving from source 43 results in resonator 11 vibrating in the reed mode whereby substantial movement of contact 48 does not occur, but engagement between contacts 44 and 45 occurs at frequency F2. Thus, lamp 54 is energized to the exclusion of lamp 55 in response to source 43 being of frequency F2.

Reference is now made to FIGURE 3 of the drawings wherein a variable frequency source 57 is coupled through electromechanical resonator 11 to load 58 at frequencies F1 and F2. At frequency F1, tuning fork 11 is vibrated in the fork mode and oscillations from source 57 are coupled directly to load 58 through coil 19 with a predetermined polarity and phase relative to the phase of the source 57. At frequency F2, resonator 11 is caused to vibrate in the reed mode, which vibrations are detected by Vauxiliary pickup coil 59, positioned to be responsive to movement of tine 13 in proximity to leg 14.

Coil 59 is wound so that the relative phase of the signal it picks up is the same as the signal picked up by coil 19 relative to source 57. Thus, if the signal deriving from coil 19 is positive when source 57 is positive, the signal derived from coil 59 is positive at the same time as when source 57 is positive. Because no phase inversion occurs in the filter between frequencies F1 and F2, the configuration of FIGURE 3 can be employed in phase detecting applications.

A further feature of filter construction according to FIGURE 3 is that the responses of the filter at both frequencies F1 and F2 can be adjusted to be substantially constant, despite the disparity of the Q for the reed and fork modes of vibration. This is accomplished by adjusting the number of turns in pickup coil 59 to be considerably greater than the number of turns in coil 19 so that the voltages generated across both coils are approximately equal in response to signals of frequency F1 and F2 being of the same amplitude.

' 0f course, in utilizing the tuning fork of the present invention as a lter, adequate shielding must be provided between coils 17, 19 and 59 to prevent magnetic coupling between them through other than tuning fork 11. If shielding is not provided, there will frequently be suicient electromagnetic coupling between the various coils to bypass the effect of the electromechanical resonator. Sutiicient shielding, for many purposes, can be obtained by enclosing each of coils 17, 19 and 59 and their associated bar magnets in cylindrical magnetic shields made of Mu metal, wherein the shields have their end pieces removed. In the alternative, for many purposes, a shield can be placed around only coil 17 and its bar magnet.

A further modification of the present invention is illustrated in FIGURE 4 Iwhere'in the input power necessary `to drive tuningfork i151 is reduced by encircling both tines 12 and 13 about drive coil 6'1. Coil 6t1 is wound about tines 12 and 13 towards their end where leg 14 is located so that engagement can be made between contacts 44 .and 45 as well as between contacts 48 and 419. DIC permanent magnet bias is provided in `the lcOniiguration of FIGURE 4 by iixedly mounting bar magnets 62 on housing 16 to which tine `13 is anchored. Bar magnet 62 can be provided with a bolt at its end so that it can serve as a magnetic bias lfor the fork as well as means for securing fork 111 in situ.

In response to a signal at frequency F1 or F2 applied to drive coil 6K1, fork 11 of FIGURE 4 oscillates in the fork or reed mode of vibration, respectively, to establish an electric circuit between either one of contacts 44, 45 or contacts 48, 49. To provide maximum efficiency in the fork of FIGURE 4, coil 61 should be positioned between point 26, FIGURE 5, and lleg 14. Thereby, opposing forces are not established in tine 12 on either side of point 26 by the magnetic force from coil 61 when resonator 1I1 vibrates in the fork mode.

While I have described and illustrated one specific embodiment of my invention, it will be clear lthat variation of the details of construction which are specifically illustrated and described may be resorted to without departing from the true spirit and scope of the invention as dened in the appended claims. For example, the etliciency of coupling energy into the tuning fork assembly of the present invention can be increased if a horseshoe magnet is substituted for bar magnet 18 in the embodiment of FIGURE 2. ASuch a bar magnet would have its north pole as illustrated in the FIGURE and its south pole in proximity to contact pad 48.

I claim:

-'1. A relay lfor deriving an output in response to two different predetermined frequencies F1 and F2 of an input signal comprising a tuning fork resonator adapted to vibrate in the tuning fork mode at frequency F1 and in the reed mode at frequency F2, said tuning fork including electric contact surface means, means responsive to said input signal for vibrating said tuning fork in the 4fork 'and reed modes at frequencies F1 and F2, respectively, and ycontact means yfor engaging said contact *surface means when said fork 'is vibrated in each of said modes.

2. The relay of claim 1 wherein said tuning fork includes a pair of tines an-d a leg joining them together, and means for anchoring one of said tines while the other tine and said leg are free to Ivibrate.

3. -The relay of claim 2 wherein said contact means includes first and second fixed contacts and said contact surface means are first and second sep-arate areas on the tuning fork, said iirst area being proximate the end of the tine free to vibrate, said second area being proximate the leg on one of said tines, said iirst contact being positioned to engage said 4tirst area only when the fork is vibrated in the fork mode, said second contact being positioned to engage said second area only when the fork isrvibrated in the reed mode.

4. An oscillator for deriving a pair of predetermined diierent frequencies F1 and F2 comprising a tuning fork resonator adapted to vibrate in the tuning fork mode at frequency F1 and in the reed mode at frequency F2, iirst velocity-position transducer means for detecting fork vibrations in the fork mode, second velocity-position transd-ucer means for detecting fork vibrations in the reed mode, an ampliiier having an input and an output, meansV for selectively coupling said first and second transducer means to said input and output, said first transducer means being coupled to said input when said second transducer is coupled to said output so said fork regeneratively vibrates in the reed mode, said second transducer means being coupled to said input when said first transducer means is coupled to said output so said fork regeneratively vibrates in the fork mode.

S. The oscillator of claim 4 wherein said tuning fork includes a pair of tine's and a lleg joining them together, and means for anchoring one of said tines while the other tine and said leg are free to vibrate.

I6. The oscillator of claim 5 wherein said firs-t transducer is positioned to be responsive to vibrations at the end of the tine free to vibrate, and said second transducer means is positioned to be responsive to vibrations of said leg.

7. An oscillator for deriving a pair of predetermined different frequencies F1 and F2 comprising a tuning fork resonator adapted to vibrate in the tuning fork mode at frequency F1 and in the reed mode at frequency F2, signal-velocity transducer means for vibrating said tuning fork in said modes, first and second signal-velocity transducer means for detecting vibrations in the fork and reed modes, respectively, an amplifier having an input and output, and means for coupling said transducer means with the input and output of said ampli-lier so said fork vibrates regeneratively in one or the other of said modes.

8. The oscillator of claim 7 wherein said vibrating transducer means selectively coincides with one of said detecting -transducer means.

9. A lter for selectively passing frequencies F1 and F2 from a variable frequency source to la load comprising a tuning fork resonator adapted to vibrate in the tuning fork mode at a frequency F1 and in the reed mode at frequency F2, signal-velocity transducer means responsive to said source for vibrating said ltuning fork in both of said modes, rst and second signal-velocity transducer means for detecting vibrations in the fork land reed modes, respectively, and means for connecting both of said detecting transducer means across said load.

10. The ilter of claim 9 wherein both of said detecting transducer means are `connected to said load to provide the same'relative phase and gain relations between the load and source voltages for both modes of tuning fork vibrations.

11. The lter of claim 9 wherein said tuning fork includes a pair of tines and a leg joining them together, and means for anchoring one of said tines while the other tine and said leg are free to vibrate.

12. The lter of claim 11 wherein said first transducer is positioned to be responsive to vibrations at the free end of the tine free to vibrate, and said second transducer means is positioned to be responsive to vibrations of said leg,

13. The iilter of claim 12 wherein both of said detecting transducer means are connected to said load to provide the same relative phase and amplitude relations between the load and source voltage for both modes of tuning fork vibrations.

14. An electromechanical resonator comprising a tuning fork having a U-shaped configuration in which normally parallel tines are connected by a bridging portion, means supporting said fork only -along one of said tines to permit freedom of vibration of said one tine along its unrestricted length and of the other tine and said bridging portion along their entire lengths, means for driving said fork in either of the tuning fork and reed modes of vibration, and separate and distinct detecting means for sensing the vibrations of said fork in the respective vibrational modes.

15. The invention according to claim 14 wherein said separate and distinct detecting means include means positioned adjacent the free end of said other tine yfor sensing the vibrations in the tuning fork mode, and means positioned adjacent said bridging portion for sensing the vibrations in the reed mode.

16. The invention according to claim 15 wherein said driving means includes at least part of one of said means for sensing.

17. The invention according to claim 14 wherein said tuning fork is composed of magnetically permeable sheet metal.

18. The invention according to claim 17 wherein said one tine is longer than said other tine, and said supporting means includes means anchoring said one tine at the segment thereof extending beyond the end of said other tine.

19. The invention according to claim 18 wherein each of said detecting means includes an electrical coil and a magnetic core extending therethrough, one of said detecting means responsive to vibrations at the free end of said other tine, and the other of said detecting means responsive to vibrations of said bridging portion, said magnetic cores providing a D-C magnetic bias for 10W path reluctance through at least a portion of said fork.

20. The invention according to claim 19 wherein said driving means includes at least the coil of one of said detecting means.

21. The invention according to claim 19 wherein said driving means comprises a variable frequency signal gcnerator for supplying an output signal at either the natural frequency of said tuning fork vibrational mode or the natural frequency of said reed vibrational mode, and an electrical coil coupled to said generator and -adjacent said fork between said free end of said other tine and said bridging portion.

22. The invention according to claim 14 wherein said fork is magnetically permeable and each of said detecting means comprises an electromagnetic pickup coil, one said coil responsive to vibrations at the free end of said other tine and another said coil responsive to vibrations of said bridging portion.

23. The invention according to claim 14 wherein each of said detecting means comprises an electrical contact for contacting a respective electrical contact on said fork only when the fork is driven in a respective one of said modes.

24. An electromechanical resonator including a tuning fork, said fork mounted only along one of its tines to permit freedom of movement thereof along the remainder of that tine, the remaining tine, and the bridge connecting the two tines; means for vibrating the fork at disparate frequencies corresponding to the natural frequencies of the tuning fork mode and the reed mode; means adjacent a segment of said fork vibrational in the tuning fork mode for detecting substantial movement of said fork in that mode; and means adjacent a segment of said fork vibrational in the reed mode for detecting substantial movement of said fork in that mode.

25. The invention according to claim 24 wherein said fork is composed of magnetically permeable material, and wherein each of said detecting means comprises an electromagnetic detector for generating a voltage proportional to the varying magnetic field accompanying vibrations of said fork.

26. The invention according to claim 25 wherein said vibrating means comprises an oscillator and means for electromagnetically coupling the output oscillations of said oscillator to said tuning fork.

27. The invention according to claim 26 wherein said coupling means comprises -at least a portion of one of said electromagnetic detector means.

28. The invention according to claim 26 wherein is further provided means for reducing the reluctance of the magnetic path between said vibrating means and each of said detecting means.

29. An electromechanical resonator comprising a tuning fork having a pair of normally parallel tines joined by a bridging member, means for vibrating one of said tines and said bridging member over their entire lengths and the other of said tines over only a portion of its length, relative to a fixed point on the fork, and means separately responsive to vibrations of said fork in the tuning fork and reed modes for detecting the respective frequencies of said vibrations.

References Cited UNITED STATES PATENTS 137,643 4/1873 Whitney 84-403 '2,026,342 12/1935 Curtiss 84-408 2,581,963 1/1952 Langloys 84-403 2,948,181 8/1960 Birkemeier 84-409 3,306,151 2/1967 Cser 84-102 L. R. FRANKLIN, Assistant Examiner. RICHARD B. WILKINSON, Primary Examiner. 

